Distance

Mass

Size

Discovery Methods

M51

One of the most beautiful of all galaxies is M51. It's nicknamed "the Whirlpool" because of its well-defined spiral arms. It actually consists of two galaxies -- the big spiral (NGC 5194), and a smaller galaxy (NGC 5195) that looks like it's being reeled in by the bigger one.

The smaller galaxy is actually moving away from the bigger one. As they move apart, their gravity is pulling out long streamers of stars that form a "bridge" between the two galaxies. The encounter has also ignited an intense outburst of starbirth in the larger galaxy. Since most of the new stars are born in the galaxy's spiral arms, they make M51's spiral structure particularly obvious.

In 1992, Hubble Space Telescope images revealed a dark "X" across the bright core of M51's larger galaxy. This X may be two disks of dust that encircle a black hole about one million times the mass of the Sun (about one-third as massive as the black hole at the heart of our own Milky Way galaxy). They form an X because we're viewing them edge-on (like viewing a Frisbee from the side instead of above). They are about 100 light-years wide, and hide the bright inner region of the disk where gas and dust are heated as they spiral in to the black hole.